diff --git a/internal/observability/field_test.go b/internal/observability/field_test.go
new file mode 100644
index 00000000..616894a1
--- /dev/null
+++ b/internal/observability/field_test.go
@@ -0,0 +1,52 @@
+package observability
+
+import (
+	"testing"
+)
+
+func TestPopFieldReturnsEmptyForNil(t *testing.T) {
+	if got := popField(nil, "key"); got != "" {
+		t.Fatalf("got %q, want empty", got)
+	}
+}
+
+func TestPopFieldReturnsEmptyForMissingKey(t *testing.T) {
+	if got := popField(map[string]any{"a": 1}, "b"); got != "" {
+		t.Fatalf("got %q, want empty", got)
+	}
+}
+
+func TestPopFieldReturnsAndDeletes(t *testing.T) {
+	m := map[string]any{"status": 200, "other": "x"}
+	got := popField(m, "status")
+	if got != "200" {
+		t.Fatalf("got %q, want %q", got, "200")
+	}
+	if _, exists := m["status"]; exists {
+		t.Fatal("popField did not delete key")
+	}
+	if m["other"] != "x" {
+		t.Fatal("popField deleted other key")
+	}
+}
+
+func TestPopFieldFormatsDuration(t *testing.T) {
+	got := popField(map[string]any{"duration_ms": 123.4}, "duration_ms")
+	if got != "123.4ms" {
+		t.Fatalf("got %q, want %q", got, "123.4ms")
+	}
+}
+
+func TestPopFieldFormatsBytes(t *testing.T) {
+	got := popField(map[string]any{"bytes": 2048.0}, "bytes")
+	if got != "2.0KB" {
+		t.Fatalf("got %q, want %q", got, "2.0KB")
+	}
+}
+
+func TestPopFieldPassthroughString(t *testing.T) {
+	got := popField(map[string]any{"method": "GET"}, "method")
+	if got != "GET" {
+		t.Fatalf("got %q, want %q", got, "GET")
+	}
+}